Some wireless communication devices, such as those based on frequency division duplexing (FDD), are designed to simultaneously transmit and receive signals using two or more frequency channels. Such simultaneous transmission and reception may inherently introduce a modulated interferer in the transmit channel frequency at the receiver input (e.g., leak-over of the signal being transmitted from a transmit chain of the device into a receive chain). Such FDD devices generally use a duplexer to isolate between the signals being transmitted and the receiver, and the leak-over at the duplexer may depend on its isolation performance in the transmit band. Traditionally, a high degree of isolation is provided at the duplexer to minimize the leak-over of the modulated transmit signal.
FDD receivers with a direct conversion architecture are generally designed with an external inter-stage filter solely to reduce the impact of the transmit leaked-over interferer signal on a first mixer within the receive chain even though there is no image frequency. This may impose a fairly severe limitation on radio receivers supporting multi-band operations, as not only is a duplexer required at each band to provide the needed transmit band isolation (e.g., of 55 dB), but an inter-stage external filter is also needed for each band. Accordingly, the support of multiple bands imposes a large number of radio frequency (RF) input/output (I/O) demands on a radio frequency integrated circuit (RFIC) integrated with a low noise amplifier (LNA) in the receive chain. In some cases, multi-band RFICs are designed with a separate chip for the LNA, which results in a multi-chip solution.
Besides the higher component count and restrictive RF I/O requirements, the transmit leak-over may force a different receive chain architecture for TDD and FDD operational modes. The use of different architectures is forced partly because of the need for an inter-stage filter in FDD designs, which may result in the use of separate receivers in devices to support both TDD and FDD operational modes. The use of separate receivers not only increases design complexity, but also uses more die area on the RFIC. Existing proposals and implementations suggest a brute force approach for FDD designs in order to remove the inter-stage filter. However, such an approach may result in higher power consumption because of a higher mixer linearity and a higher level of complexity in calibration needed in smaller line width processes (e.g., 90 nm or less) due to larger variations in order to ensure performance in the presence of the transmit leak-over signal.
Accordingly, an improved system and method for transmitter leak-over cancellation are needed.